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m m 1SFS/6 



THE 0UEEN5 AWARD 



RESEARCH DEPARTMENT REPORT 



Epipment for the transfer of 
digital video signals to paper tape 



C. K.P.Clarke 



Research Department, Engineering Division 

THE BRITISH BROADCASTING CORPORATION June 1973 (PH-108) 



BBC RD 1973/6 

UDC 621.376.56 
621.397 
681.3.074 



EQUIPMENT FOR THE TRANSFER OF DIGITAL VIDEO SIGNALS TO PAPER TAPE 

C.K.P. Clarke 



Summary 

A digital computer can be used to simulate processing methods for the bit-rate 
reduction of digital television. To assess such methods, television signals must be con- 
verted to a form compatible with computer data-handling systems. This report describes 
equipment for transferring monochrome pulse-code modulation television signals derived 
from a slide scanner to punched paper tape. One eight-bit word is extracted from the 
repeated signal during each picture period and is then punched as a single eight-element 
character on the tape. 

The equipment has been used to produce two paper tapes each containing an 
area of picture 256 picture elements wide and 256 lines high. The report includes a 
picture reconstituted from one of the tapes. 

Because the phase of the colour subcarrier at a point in the picture is the same 
only once every four picture periods, the equipment must be modified to take samples at 
this rate, if it is to be used with coded colour signals. More extensive modification would 
be needed if subcarrier-locked sampling were to be used. 



Issued under the authority of 




Oa^^ 



Research Department, Engineering Division, 

BRITISH BROADCASTING CORPORATION Head of Research Department 

(PH-108) 



BBC RD 1973/6 
EQUIPMENT FOR THE TRANSFER OF DIGITAL VIDEO SIGNALS TO PAPER TAPE 

Section Title Page 

Summary Title Page 

1. Introduction 1 

2. Principle of operation 1 

3. Method of testing . . . , ; 3 

4. Modifications for colour signals 3 

5. Conclusions 3 

6. Reference , 3 



(PH-108) 



June 1973 



BBC RD 1973/6 

UDC 621.376.56 
621.397 
681.3.074 



EQUIPMENT FOR THE TRANSFER OF DIGITAL VIDEO SIGNALS TO PAPER TAPE 

C.K.P. Clarke 



1. Introduction 

A general-purpose digital computer may be programmed 
to perform exactly the same arithmetical and logical 
operations as a processor for the bit-rate reduction of 
digital television. However, such computers are not 

sufficiently fast to accept and process television signals in 
real-time. So, in order to simulate a bit-rate reduction 
method by using a digital computer, it is necessary to 
reduce the input data rate by converting the signal to a 
form which may be readily accepted by a computer, for 
example, as characters punched on paper tape. Also, the 
signals processed by the computer must be stored and sub- 
sequently read out at television rate onto a picture 
monitor. 



slide 
scanner 



monochrome 
video 



a.d.c. 



mixed 
syncs 



line locked 
clocks 



sprocket punch 
and feed 



8-bit p.c.m 
video 



v: 



sompler 



\y 



8-blt sample 
words 



paper tape 
punch 



In this way, many of the degrading effects introduced 
by the bit-rate reduction process may be seen and assessed 
without building a specialised processor. However, because 
it is impracticable to provide sufficient storage to hold 
several frames of processed signal, it is not possible to 
assess the effects produced on moving pictures with this 
simulation method. 



Fig. 1 - Arrangement for transferring digital video signals 
to paper tape 

a further counter, advanced once per picture, is used to 
register the number of samples taken. The outputs of these 
counters are compared as shown in Fig. 2 and a sample is 
taken when the outputs are identical. As a result, the 2'* 



To provide suitable input data, equipment has been 
developed to transfer eight-bit pulse-code modulation tele- 
vision signals to punched paper tape. The equipment 
transfers a square section of picture consisting of 2'^ 
(65536) picture elements, that is approximately one-sixth 
of the area of a television picture. With a punch operating 
at 25 characters per second, the transfer takes 44 minutes 
and uses 170 metres of paper tape. 



2. Principle of operation 

The rate of signal transfer to the paper-tape punch is 
reduced by sampling a repeated picture signal. The arrange- 
ment used Is shown in Fig. 1, where the slide scanner 
provides stationary monochrome picture signals which are 
converted to eight-bit digital form by the Analogue-to- 
Digital Converter,^ and the sampler selects one word during 
each picture period and operates the paper-tape punch to 
produce a corresponding eight-hole character. 

The sampler contains two counters which establish 
a co-ordinate system for the picture. The horizontal 
counter is advanced at 851 times line-frequency and reset at 
the beginning of each line, whilst the vertical counter runs 
at line rate and is reset at the beginning of each field. A 
picture-rate square wave is used to distinguish the fields and 



odd/even field 
square wave 



horizontal 
counter 



LSB >' 



— 8-bits--- 



>' MSB 



vertical 
counter 



LSB \f 



--7-bit5--- 



>'MSB 



comparator 



LSB 



-16-bils 



sampling 
pulse 



MSB 



sample counter 



Fig. 2 - Generation of sample position 



(PH-108) 







■s 

.o 
.til 



BBC RD 1973/6 



- 2 



states of the sample counter correspond to 65536 different 
positions on the picture. By comparing bits 1 to 8 of the 
sample counter with the horizontal counter, bit 9 with the 
Odd/Even-field square wave and bits 10 to 16 with the 
vertical counter, an area 256 picture elements wide and 256 
lines high is canned sequentially. A sequential scan was 
chosen in preference to an interlaced scan to minimise the 
effect of drift in the analogue circuits of the slide scanner 
and analogue-to-digital converter during the scanning period. 
The full logic diagram of the sampler is shown in Fig. 3. 

Whilst scanning along any line, the interval between 
sampling pulses is two field periods. However, because a 
sequential scanning order has been chosen, the interval 
between the last sample of a line and the first sample of the 
next line varies from just over one field period to just over 
three field periods. In order to provide a regular flow of 
data to the punch, the eight-bit words must be stored in a 
second register at the end of each picture and read from 
this to the punch. 

Although the horizontal counter tal<es up 851 dif- 
ferent states in a line period, samples are only taken over 
the 256 lowest states. By resetting the counter to an 
appropriate state at the beginning of each line, the sampled 
area can be positioned anywhere along a line. A similar 
method is used to determine the vertical position of the 
sampled area. 

Operation of the sampler is controlled by a switch 
having two positions: Run and Reset. The switch is 
latched to ensure correct starting and the latching circuits 
automatically stop the sampler after 2'* samples have been 
taken. 

The system produces paper tapes on which the data 
appears as a continuous block of 2'* characters. Since 
samples are only taken during the active line, there is no 
possibility of null characters (no holes punched) being 
produced as data and the section of lead-in tape can be 
recognised. 



3. Method of testing 

Although it was not possible to check the picture 
information stored on the tapes directly, tests were made 
to establish that the correct samples were punched. The 
sampling order was checked by observing the traverse of the 
sampling pulse on a picture monitor. Paper tapes made 
from analogue waveforms of known amplitude and timing 
were examined visually to check that the correct sample 
values were transferred; further, an electronic counter was 
used to count the number of characters punched on full- 
length tapes. No errors were detected. 

Two paper tapes of picture signals have been pro- 
duced, one corresponding to a portrait and the other to a 
section of test card. The portrait comprises both plain 
areas and areas of fine detail, whilst the test card includes 



horizontal, vertical and diagonal bars. Fig. 4 shows the 
portrait* reproduced on a computer line-printer using a 
sixteen level grey-scale of variable character density. 



4. Modifications for colour signals 

The equipment as described cannot satisfactorily be 
used with coded colour signals because the phase of the 
colour subcarrier at a point in the picture is the same only 
once every four pictures. Thus, to reproduce a portion of 
a coded colour signal, the samples must be taken from every 
fourth picture. To prevent the transfer taking four times 
as long, a store could be used to hold four consecutive 
samples from one picture which would then be punched at 
the rate of one per picture (the maximum rate at which the 
paper-tape punch can operate). 

If it were intended to decode the colour signals as part 
of the computer processing, it would be much more con- 
venient to use samples phase-locked to the subcarrier than 
the line-locked samples used for monochrome transfers. 
However, subcarrier-locked sampling would require a con- 
siderably more complicated horizontal counter because line 
pulses could no longer be used for resetting the count. 
Furthermore, extra samples, conveying the subcarrier 
reference phase on each line, would be required. 



5. Conclusions 

The monochrome transfer equipment establishes a 
system of positional co-ordinates for a television picture 
851 picture elements wide and 625 lines high. Any area of 
the picture 256 picture elements wide and 256 lines high 
can be scanned sequentially and this section of the picture 
signal transferred in digital form onto punched paper tape. 
Each picture element, represented digitally as an eight-bit 
word, is punched as a single eight-digit character on the 
tape. Two paper tapes of picture signals suitable for testing 
bit-rate reduction techniques have been made, one describ- 
ing a portrait and the other a section of test card. 

The design of the equipment could be easily modified 
to accommodate coded colour signals provided that samp- 
ling were still locked to a multiple of line frequency. How- 
ever, to transfer coded colour signals in a convenient form 
for subsequent decoding, it would be necessary to use 
sampling locked to the colour subcarrier; this would entail 
extensive changes to the counter determining the horizontal 
co-ordinate. 



6. Reference 

1. Pulse code modulation of video signals: 8-bit coder 
and decoder. BBC Research Department Report No. 
1970/25. 

• Reproduced by kind permission of Professor P.S. Brandon, 
Cambridge University. 



BBC RD 1973/6 



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